Catalysts comprising a phosphorus modified zeolite (the phosphorus modified zeolite is also referred as P-zeolite) are known. The following prior arts have described various methods to make said catalysts.
US 2006 106270 relates to the use of a dual-function catalyst system in the hydrocarbon synthesis reaction zone of an oxygenate to propylene (OTP) process that operates at relatively high temperatures preferably with a steam diluent and uses moving bed reactor technology. The dual-functional catalyst system comprises a molecular sieve having dual-function capability dispersed in a phosphorus-modified alumina matrix containing labile phosphorus and/or aluminum anions. It is explained that the hydrothermal stabilization effect that is observed when this phosphorus-modified alumina matrix is utilized is caused by migration or dispersion of phosphorus and/or aluminum anions from this matrix into the bound molecular sieve. These anions are then available to repair, anneal and/or stabilize the framework of the molecular sieve against the well-known dealumination mechanism of molecular sieve framework destruction or modification that is induced by exposure to steam at temperatures corresponding to those used in the OTP reaction zone and in the regeneration zone.
U.S. Pat. No. 4,356,338 discloses a method for decreasing catalyst coking and extending the usable catalyst life by pre-treatment of the catalyst with steam and/or a phosphorus-containing compound. Pretreatment may be accomplished by the impregnation of the catalyst or of the catalyst/binder combination with a phosphorus containing compound to deposit approximately 4 wt. % of phosphorus thereon, and preferably from about 2% to about 15% by weight of phosphorus, based on the weight of the catalyst or catalyst/binder matrix being treated.
U.S. Pat. No. 5,231,064 is directed to a fluid catalyst comprising clay and a zeolite, at least one of which has been treated with a phosphorus containing compound, for example ammonium dihydrogen phosphate or phosphoric acid, and which is spray dried at a low pH, preferably lower than about 3. Said catalysts are deemed to advantageously exhibit reduced attrition.
EP 511013 A2 provides an improved process for the production of C2-C5 olefins from higher olefinic or paraffinic or mixed olefin and paraffin feedstocks. In accordance with this prior art, the hydrocarbon feed materials are contacted with a particular ZSM-5 catalyst at elevated temperatures, high space velocity and low hydrocarbon partial pressure to produce lower olefins. The catalysts is treated with steam prior to use in the hydrocarbon conversion. The active catalyst component is phosphorus-containing ZSM-5 having a surface Si/Al ratio in the range 20-60. Preferably, the phosphorus is added to the formed ZSM-5 as by impregnating the ZSM-5 with a phosphorus compound in accordance with the procedures described, for example, in U.S. Pat. No. 3,972,832. Less preferably, the phosphorus compound can be added to the multicomponent mixture from which the catalyst is formed. The phosphorus compound is added in amount sufficient to provide a final ZSM-5 composition having 0.1-10 wt. % phosphorus, preferably 1-3 wt. %.
The phosphorus-containing ZSM-5 is preferably combined with known binders or matrices such as silica, kaolin, calcium bentonite, alumina, silica aluminate and the like. The ZSM-5 generally comprises 1-50 wt. % of the catalyst composition, preferably 5-30 wt. % and most preferably 10-25 wt. %.
EP 568913 A2 describes a method for preparing a ZSM-5 based catalyst adapted to be used in the catalytic conversion of methanol or dimethyl ether to light olefins, wherein it comprises the following consecutive steps:                mixing a zeolite ZSM-5 based catalyst with silica sol and ammonium nitrate solution,        kneading, moulding, drying and calcining the mixture,        exchanging the modified zeolite with a solution of HCl at 70-90° C.,        drying and calcining the H-modified zeolite,        impregnating the H-modified zeolite with phosphoric acid under reduced pressure,        drying and calcining the P-modified zeolite,        impregnating the P-modified zeolite with a solution of rare earth elements under reduced pressure,        drying and calcining the P-rare earths-modified zeolite,        hydrothermally treating the P-rare earths-modified zeolite at 500-600° C. with water vapour, and        calcining the modified zeolite.        
WO 03 020667 relates to a process of making olefin, particularly ethylene and propylene, from an oxygenate feed, comprising contacting an oxygenate feed with at least two different zeolite catalysts to form an olefin composition, wherein a first of the zeolite catalysts contains a ZSM-5 molecular sieve and a second of the zeolite catalysts contains a zeolite molecular sieve selected from the group consisting of ZSM-22, ZSM-23, ZSM-35, ZSM-48, and mixtures thereof. The ZSM-5 can be unmodified, phosphorous modified, steam modified having a micropore is volume reduced to not less than 50% of that of the unsteamed ZSM-5, or various mixtures thereof. According to one embodiment, the zeolite is modified with a phosphorous containing compound to control reduction in pore volume. Alternatively, the zeolite is steamed, and the phosphorous compound is added prior to or after steaming. The amount of phosphorous, as measured on an elemental basis, is from 0.05 wt. % to 20 wt. %, and preferably is from 1 wt. % to 10 wt. %, based on the weight of the zeolite molecular sieve. Preferably, the atomic ratio of phosphorus to framework aluminum (i.e. in the zeolite framework) is not greater than 4:1 and more preferably from 2:1 to 4:1. Incorporation of a phosphorus modifier into the catalyst of the invention is accomplished, according to one embodiment, by contacting the zeolite molecular sieve either alone or the zeolite in combination with a binder with a solution of an appropriate phosphorus compound. The solid zeolite or zeolite catalyst is separated from the phosphorous solution, dried and calcined. In some cases, the added phosphorous is converted to its oxide form under such conditions. Contact with the phosphorus-containing compound is generally conducted at a temperature from 25° C. to 125° C. for a time from 15 minutes to 20 hours. The concentration of the phosphorus in the zeolite may be from 0.01 wt. % to 30 wt. %. This prior art discloses a non-formulated P-ZSM-5.
A common way to produce a formulated P-zeolite containing catalyst consists in the impregnation of the already pre-formulated zeolite (e.g. the zeolite+ a binder) with P-compounds or phosphorous addition to the reaction medium.
A great number of patents disclose the recipe for preparation of the active phase (non-formulated phosphated zeolite) by means of zeolite phosphatation and their use in methanol conversion. Some of these references contain the options of further blending the active phase with binder. However, the active phase is good as such in the reaction. It is assumed that the binder plays only the role of diluent what is not normally the case. The process of the present invention differs from a great number of known in the art preparation of the P-zeolite based active phase due to referring to preparation of formulated catalyst and implementation of the phosphatation step at the first stage. Moreover the phosphatation of the zeolite (formation of the active phase) at the first step does not necessarily leads to a suitable catalyst. On the contrary, the overall recipe results in a good catalyst.
The catalyst referred to in the present invention comprises a zeolite and at least a component selected among one or more binders, salts of alkali-earth metals, salts of rare-earth metals, clays and shaping additives. The metal salts, binder and clays may also adsorb the phosphorous interfering and even competing with zeolite preventing a proper zeolite phosphatation. The presence of traces of metals adsorbing preferentially phosphorous could even more perturb the zeolite phophatation. This often leads to non-selective catalysts due to poor reproducibility and binder pore plugging. The method of the present invention provides a solution to selectively phosphatize zeolite overcoming the side effects of binder, metal salts or clays presence. Thus, the invention discloses that the preparation of the catalyst requires the phosphatation of zeolite before introducing any other components such as binder, metals, clays and shaping additives. This method insures the reproducibility of the preparation, the hydrothermal stability and the good catalyst performance.